Acoustic device



March 27, 1956 F. B. DANIELS 2,739,659

ACOUSTIC DEVICE Filed Sept. 5, 1950 FIG. 4

INVENTOR. FRED B. DANIELS FIG.5

United States Patent ACOUSTIC DEVICE Fred B. Daniels, Red Bank, N. J.,assignor to the United States of America as represented by the Secretaryof the Army The invention described herein may be manufactured and usedby or for the Government for governmental purposes without the paymentof any royalty thereon.

This invention relates to an electro-acoustical apparatus and moreparticularly to a device for translating acoustical waves intoelectrical variations, and includes an acoustical transmission line ofnovel design. This invention relates particularly to a highlydirectional device that is relatively insensitive to interference fromwind and other air turbulences.

There are several types of acoustical impedance elements for use inconjunction with microphones, those shown in Patent No. 2,228,886 to H.F. Olson being representative of the existing types that most nearlyresemble this invention and which generally consist of a plurality ofinput tubes of varying length coupled to a common electro-acousticaltransducer and terminating in some sort of acoustical impedance.

It is an object of this invention to provide an acoustical couplingdevice for an electro-acoustical transducer.

It is a further object of this invention to provide an acousticalcoupling device that is highly directional.

It is a further object of this invention to provide an acousticalcoupling device that does not require an additional terminatingacoustical impedance, for use with an electro-acoustical transducer.

It is a further object of this invention to provide an acousticalcoupling device that is relatively compact and easily constructed.

This device is particularly valuable in sound ranging systems where theeddy characteristics of the wind at the electro-acoustical transducerclosely resemble the muzzle wave that it is desired to detect.

The invention and the foregoing as well as other features thereof willbe understood more clearly from the following detailed description andwith reference to the accompanying drawings in which:

Figs. 1 to 4 show sectional views through the axis of various species ofthis invention, and

Fig. 5 shows a projection of one exterior surface of another species.

Referring now to the drawings, wherein similar reference numbersdesignate corresponding parts throughout, Fig. 1 shows an acousticalcoupling device coupled to an electro-acoustical transducer 11 in achamber 12. The acoustical coupling device 10 which is particularlydesigned to have directional and other desirable features, here consistsof a series of tubular pipes 13, 14, 15 and 16, one end of eachconnecting to the chamber 12 and the other end of each terminating inplugs 17, 18, 19 and 20. These plugs have vents 21, 22, 23 and 24 of thesize and shape necessary to match the characteristic impedance of theirrespective pipes. Since each pipe is, then, terminated by itscharacteristic impedance at the input end, no additional terminatingimpedance beyond the chamber 12 is required. Without these plugs ofprescribed impedance, a terminating impedance such as a damped pipe,would be necessary, and this would add considerably to 2 the length,weight and cost of manufacture of the device. Besides, the commonly useddamping materials-that such a damped pipe would require-might absorbmoisture, become displaced, or otherwise be rendered ineffectual.

A preferred form of this invention is seen in Fig. 2

where the pipes 1316 of Fig. 1 are replaced by a single pipe ofsuccessively increasing sections 113, 114, and 116. Plugs 117-420 aremounted near the junctions between the sections, and the holes 121-424in these plugs are, again, of suitable dimensions to match each sectionof the pipe individually and as a whole. This impedance matching isachieved by making 1 1 win-st) contains an electro-mechanical device111, such as a conventional pressure microphone. If physical dimensionspermit, the device 111 may be mounted directly in the end of the pipeand the chamber 112 may be replaced by a flat cover.

This device is particularly suited to the pressure type microphone,which is cheaper, more rugged, and more suitable for low frequenciesthan the velocity type microphone.

In operation, the sound waves entering the openings 121124 arereinforced in the direction of the axis of the pipe, since, for example,a sound pulse traveling through the air along the outside of the pipewill enter through each successive opening, 121-124, exactly in phasewith pulse traveling through the pipe, while the sound waves approachingfrom other angles will be more or less neutralized by the phasedifference between a pulse traveling through the inside of the pipe andthe successive pulses from the openings produced by the same wave on theoutside of the pipe, thus making the device directional.

The apparatus of Fig. 2 can be easily and economically constructed sinceit can be made of ordinary water pipe of successively increasing sizes.This device can be made of considerable size. For example, one devicewas made 1980 feet long with 99 openings. Also, it may be paralleledwith similar devices.

The wind disturbances in the form of eddy currents and other localizedpressure variations are minimized to a large degree by this device sincethe wind interference is local and comparatively static while a soundwave exists only in motion. The effect of the wind is local and randomWhile a sound wave arrives at the various openings with a definite phaserelationship. A local pressure variation effecting only a few openingswould have a cumulative effect negligible compared to that of a soundwave coming from the right direction and acting on all of the openings.This noise reducing eitect is very valuable in sound ranging where theeffect of a local pressure change due to wind currents on a microphoneclosely resembles the pressure change due to a muzzle wave.

Another variation of this device, shown in Fig. 4 would have holes321-324 drilled in the papes 313-316 and the pipe lengths or hole sizesand numbers chosen to meet the basic requirement of this invention, thatthe openings and pipes form a balanced acoustical unit not requiringadditional termination.

Another species of this invention is seen in Fig. 3

where the coupled sections of pipe of increasing diameter are replacedby a parabolic horn 210 of equivalent length and volume. This would beeasily accomplished for relatively short structures. In this species,the plugs 217-220 and end openings 221-224 could be replaced by drilledholes of similar impedance as in Fig. 4, or a continuous slot 425, shownin Fig. 5, whose impedance per unit length balances the volumetricchange so that the pipe 410 can be coupled to the electro-acousticalmember without additional acoustical termination. In this case thereciprocal of the acoustical resistance of the slot per unit length mustbe equal to the change per unit length of the-reciprocal of thecharacteristic impedance of the horn.

What is claimed is:

1. An acoustical pipe comprising; a plurality of pipe sections havingopen ends and solid walls of discrete internal diameters, axiallycoupled in order of increasing diameter, each of said pipe sectionshaving orifices of acoustical resistance which matches the change inacoustical impedance of the pipe sections adjacent to said orifices, theterminating ends of said acoustical pipe being acoustically sealed; andan electro-acoustical transducer mounted in the larger of said pipesections.

2. An acoustical transducer comprising; an enclosure of increasingcross-sectional area having a series of perforations in said enclosuresubstantially in the direction of the increase in said cross-sectionalarea, the reciprocal of the acoustical resistance of each perforationequal to times the increase in cross-sectional area of said enclosurebetween said perforation and the last adjacent perforation, where d isthe density of air and c is the velocity of sound expressed in c. g. s.units.

3. An acoustical transducer comprising a first enclosure of decreasingcross-sectional area having a plurality of openings in said enclosurespaced in the direction of decreasing area, each of said openings havingthe reciprocal of its acoustical resistance equal to times the decreasein area between the cross-sectional area of said enclosure at saidopening and the crosssectional area of said enclosure at the adjacentopening where d is the density of air and c is the velocity of soundexpressed in c. g. s. units, and a second enclosure containing anelectromechanical transducer terminating the larger end of said firstenclosure.

4. An acoustical transducer as in claim 3, wherein said first enclosurecomprises a series of pipe segments of decreasing cross section coupledaxially, and said openings are situated at each successive coupling.

5. An acoustical transducer as in claim 3, wherein said first enclosureconsists of a tubular member of continuously decreasing cross section.

6. An acoustical transducer as in claim 3, wherein said first enclosureconsists of a parabolically tapered pipe.

7. An acoustical transducer as in claim 3, wherein each of said openingsconsist of a plurality of cylindrical apertures in the casing ofsaid-first enclosure.

8. An acoustical transducer as in claim 3, wherein said openings areformed in plugs having centrally located tubular openings extendingthrough said plugs and said plugs extend through the casing of saidfirst enclosure.

References Cited in the file of this patent UNITED STATES PATENTS1,715,831 Hahnemann July 4, 1929 2,122,447 Zand July 5, 1938 2,210,415Kellogg Aug. 6, 1940 2,228,886 Olson Jan. 14, 1941 2,293,181 Terman Aug.18, 1942 2,297,046 Bourne Sept. 29, 1942 2,299,342 Olson Oct. 20, 19422,514,344 Slaymaker et al July 4, 1950

